Re: [PATCH] ARM: disallow ARM_THUMB for ARMv4 builds

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 16 December 2016 at 09:14, Arnd Bergmann <arnd@arndb.de> wrote:
> With old compilers (gcc-4.3 and earlier), we run into a build error
> when CONFIG_ARM_THUMB is enabled in kernels that have both ARMv4 and
> ARMv4T/ARMv5 CPU support:
>
> arch/arm/kernel/entry-armv.S: Assembler messages:
> arch/arm/kernel/entry-armv.S:938: Error: selected processor does not support `bx lr' in ARM mode
> arch/arm/kernel/entry-armv.S:961: Error: selected processor does not support `bx lr' in ARM mode
> arch/arm/kernel/entry-armv.S:1004: Error: selected processor does not support `bx lr' in ARM mode
>
> The problem evidently is that 'bx' cannot work on the old CPUs, but
> the new ones have to use it whenever returning to THUMB user space.
>

Can't we use the old

tst lr, #1
moveq pc, lr
bx lr

trick? (where bx lr needs to be emitted as a plain opcode to hide it
from the assembler)

> This was discussed a while ago without a conclusion about what
> the proper patch should be to solve it, and came again up now when I
> experimented with old toolchain versions.
>
> This sidesteps the problem by declaring that we do not support
> the configuration and instead have to disable CONFIG_ARM_THUMB
> and not use THUMB user space with a kernel that supports the
> FA526 CPU, which is the only one that is allowed in a multiplatform
> configuration together with ARMv4T/ARMv5 anyway. This is not
> a regression because the configuration never worked anyway.
> The only platform affected by this is moxart, as no other ARMv4
> platforms are part of ARCH_MULTIPLATFORM.
>
> Cc: Jonas Jensen <jonas.jensen@gmail.com>
> Link: http://lkml.iu.edu/hypermail/linux/kernel/1404.1/00908.html
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/arm/mm/Kconfig | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
> index 5d4920c069f2..f7b7aa37964f 100644
> --- a/arch/arm/mm/Kconfig
> +++ b/arch/arm/mm/Kconfig
> @@ -655,11 +655,7 @@ config ARCH_DMA_ADDR_T_64BIT
>
>  config ARM_THUMB
>         bool "Support Thumb user binaries" if !CPU_THUMBONLY
> -       depends on CPU_ARM720T || CPU_ARM740T || CPU_ARM920T || CPU_ARM922T || \
> -               CPU_ARM925T || CPU_ARM926T || CPU_ARM940T || CPU_ARM946E || \
> -               CPU_ARM1020 || CPU_ARM1020E || CPU_ARM1022 || CPU_ARM1026 || \
> -               CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_V6 || CPU_V6K || \
> -               CPU_V7 || CPU_FEROCEON || CPU_V7M
> +       depends on !(CPU_32v3 || CPU_32v4)
>         default y
>         help
>           Say Y if you want to include kernel support for running user space
> --
> 2.9.0
>
>
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